Method for determining rheological parameters of a fluid

ABSTRACT

The inventive method uses the ultrasound Doppler method (UVP) in order to determine a local velocity profile perpendicular to a line for a fluid which flows through said line, carrying suspended or emulsified particles. The wall shear stress of said fluid is measured locally within the range of said local velocity profile. Specific rheological parameters of the flowing fluid thus examined, e.g. viscosity function (shear viscosity), flow limit etc., can be determined from the local velocity profile and the local wall shear stress associated therewith. A suitable model is adapted by iteratively adapting a model-based theoretic velocity profile to a measured velocity profile.

This invention relates to a method and device for determining the rheological parameters of a flowing fluid, in particular of a suspension or an emulsion, according to claim 1 or claim 24.

Similar methods and devices are already known. The ultrasound Doppler method is here used to determine a local velocity profile perpendicular to a line for the fluid that flows through a flow channel and carries suspended or emulsified particles. In addition, the static pressure is measured upstream and downstream from the area of the determined local velocity profile to determine a pressure difference along the direction of flow in the area of the local velocity profile. The velocity profile and the associated pressure difference profile can then be used to determine specific rheological parameters of the examined flowing fluid, e.g., the viscosity function (shear viscosity), yield point, etc.

This combination of ultrasound Doppler method (UVP, ultrasound velocity profiling) and pressure difference determination (PD, pressure difference), referred to by experts as UVP-PD for short, has been described in numerous publications in different variants, always with slight modifications.

The article “Velocity profile measurement by ultrasound Doppler shift method” by Y. Takeda in the International Journal of Heat and Fluid Flow”, Vol. 7, No. 4, December 1986, confirms the suitability of the UVP method for determining a one-dimensional velocity profile in tubules or blood vessels only several millimeters in diameter.

The “Rheological Study of Non-Newtonian Fluids” by E. Windhab, B. Ouriev, T. Wagner and M. Drost, 1^(st) International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering, September 1996 describes the aforementioned UWP-PD method.

“Ultrasound Doppler Based In-Line Rheometry of Highly Concentrated Suspensions” by B. Ouriev, Diss. ETH No. 13523, Zürich 2000, contains an extensive description of the theoretical and equipment-related principles of the UVP-PD method along with its application in drag shear flows and pressure shear flows, in particular of model suspensions or given rheological fluids, e.g., during the manufacture of chocolate or pasta products. This publication deals with both laminar and turbulent flows.

The UVP-PD method described here yields good results for different kinds of velocity profiles and for the rheological parameters to be determined for the examined fluids. However, the two spaced apart measuring points are always required for measuring the pressure upstream and downstream from the determined velocity profile. While use can today be made of slightly intrusive miniaturized ultrasound transceivers, ultrasound measuring transducers and pressure sensors, the distance between the two pressure measuring points always required upstream and downstream from the acquired local velocity profile for reasons of measuring accuracy already places limits on a further “compacting” of the UVP-PD measuring device by edging the two pressure sensors together.

The object of the invention is to provide a method and measuring device that uses the ultrasound Doppler method and enables a more compact measuring device along with a simplification of the method complexity relative to the UVP-PD method of prior art.

This object is achieved by the method according to claim 1 and the device according to claim 24.

The method according to the invention for determining the rheological parameters of a flowing fluid, in particular of a suspension or emulsion, requires that the fluid flow be limited at least in some areas by a wall contacting the fluid, and involves the following steps:

-   -   a) Sending into the fluid flow an ultrasound signal transmitted         from an ultrasound transmitter at least at one prescribed first         frequency f1 at an angle θ differing by 90° relative to the         direction of flow;     -   b) Receiving into an ultrasound receiver an ultrasound signal         reflected by particles entrained in the fluid in respective         fluid areas with at least one second frequency f2 that is         characteristic for the respective fluid area and shifted         relative to the frequency f1 by a respective frequency shift Δf;     -   c) Acquiring the local wall thrust stress in at least an area of         the fluid contacting the wall;     -   d) Calculating the at least one frequency shift Δf using the at         least one first frequency f1 and the at least one second         frequency f2;     -   e) Allocating the respective frequency shift Δf to a respective         fluid area using the respective running time of the ultrasound         signal between the point of transmission from the ultrasound         transmitter and point of reception by the ultrasound receiver;     -   f) Calculating the fluid velocity of the respective fluid area         in which the reflecting particles are entrained using the         respective frequency shift;     -   g) Calculating rheological parameters of the fluid using the         wall thrust stress of the fluid acquired in the at least one         local wall area and the calculated fluid velocity of the         respective local fluid areas of the flowing fluid.

The arrangement according to the invention for determining the rheological parameters of the flowing fluid using the method according to the invention consists of:

-   -   At least one wall area that contacts and borders a fluid flowing         in the device at least in partial areas of the fluid interface;     -   An ultrasound transmitter for sending an ultrasound signal with         at least one preset frequency f1 at an angle θ differing by 90°         relative to the direction of flow of a fluid flowing in the         device;     -   An ultrasound receiver for receiving an ultrasound signal with         at least one frequency f2 that can be shifted relative to         frequency f1 by a frequency shift Δf;     -   At least one thrust stress sensor for detecting a wall thrust         stress in at least one area of the fluid in contact with the         wall;     -   One computer and processor for calculating frequency differences         and allocating a specific frequency difference to a respective         fluid area using a respective time difference between the         transmission and reception of an ultrasound signal; for         calculating the fluid velocity of a respective fluid area using         the respective frequency shift; and for calculating rheological         parameters of the fluid using the acquired wall thrust stress of         the fluid and the calculated fluid velocity of the respective         fluid areas of the flowing fluid.

The two pressure measurements in prior art are avoided by acquiring the local wall thrust stress in step c), i.e., by performing a single thrust stress measurement in the area of the boundary layer of the flowing fluid. This makes it possible to reduce the amount of space and cabling required for the measuring arrangement, and also to simplify the method.

Since the method according to the invention requires that the fluid flow be limited at least in partial areas by a wall in contact with the fluid, the method according to the invention can also be applied to fluid flows in a partially open channel, e.g., in a fluid flow driven by gravitational force in an inclined groove, or a fluid flow between the cylinder jacket of the rotor and stator, e.g., in a rotational rheometer. These applications are also facilitated by the compact measuring arrangement for the method according to the invention. It is completely sufficient to examine the behavior of the flowing fluid (fluid velocities in local partial areas) in a local area that accommodates the ultrasound transmitter, the ultrasound receiver or, if necessary, an ultrasound transceiver and the thrust stress sensor are located during exposure of the fluid to external influences (thrust effect, e.g., owing to pressure differences or inertia forces; drag effect of a fluid that adheres to a moving wall or slides along it with slippage). This makes the method suitable for studying the thrust and drag-shear flows, as well as combined thrust-drag-shear flows. However, turbulent flows can also be analyzed.

In shear flows, which are kept going by such a wall-shear effect, e.g., in the rotational rheometer of the preceding paragraph, it is even impossible to use the UVP-PD method of prior art, since, while fluid velocities can be determined via UVP, pressure drops along the direction of flow cannot.

By contrast, the method according to the invention in conjunction with the arrangement according to the invention enables the use of UVP both in pure drag-shear flows, mixed driven drag/thrust-shear flows, in particular in tubular flows with inner stopper, and naturally in pure thrust-shear flows based on the local, in extreme cases even point, measurement of the wall thrust stress in the boundary layer area of the fluid. Any experimental “boundary conditions” can hence be selected. This opens up new capabilities for the parameterization of different flows and allocation between such parameters and continuum-mechanical or microscopic, particle-based models.

The transmitted ultrasound signal can be a signal with several discrete first frequencies (f1, f1′, f1″, . . . ), and the received ultrasound signal can be at least a second signal, each with several discrete second frequencies (f2, f2′, f2″, . . . ), which are shifted relative to the respective first frequencies (f1, f1′, f1″, . . . ) by a respective frequency shift Δf characteristic for the respective fluid area. This makes it possible to determine the fluid velocity characteristic for a fluid area based on several subtraction operations, for which f2−f1=f2′−f1′=f2″=f1″= . . . =Δf, at least in first approximation. The arithmetic mean is preferably found for the individual differences to obtain a reliable value for Δf, and hence for the respective fluid velocity of one of the fluid areas.

The transmitted ultrasound signal can also be a signal with a first frequency spectrum (FS1), and the received ultrasound signal can be at least one second signal with a respective second frequency spectrum (FS2), which is shifted relative to the first frequency spectrum (FS1) by a respective frequency shift Δf characteristic for the respective fluid area. Frequency shifts can here also be used at several points in the two frequency spectra for averaging the value Δf, and hence determining the respective fluid velocity of one of the fluid areas.

Pulsed signals are preferably used for the transmitted, and hence also for the received ultrasound signals. In step e) of the method according to the invention, this simplifies the allocation of the respective frequency shift Δf and a respective fluid area using the respective running time of the ultrasound signal between the time it leaves the ultrasound transmitter and is received by the ultrasound receiver. In particular, the pulsed signals here each have a constant carrier frequency.

The transmitted and received ultrasound signals can also each be continuous signals, however. This is advantageous in particular when using the frequency spectra FS1 and FS2 described further above.

The wall thrust stress is best acquired only in a single area of the fluid in contact with the wall. This enables a particularly compact realization of the method according to the invention.

The ultrasound signal radiated into the fluid is best transmitted and the reflected ultrasound signal is best received at the same location, e.g., by means of an ultrasound transceiver.

A local velocity profile is preferably established transverse to the direction of flow using the fluid velocities of the fluid areas of the fluid calculated in step f), wherein the viscosity function (shear viscosity) of the fluid is determined in particular from the local velocity profile calculated in step f) and the local wall thrust stress acquired in step c) in at least one area of the fluid in contact with the wall.

In the method according to the invention, a suitable model is preferably tailored by iteratively adjusting a model-based theoretic velocity profile to a measured velocity profile. Diverse rheological parameters can then be derived from the adjusted theoretical velocity profile.

The measured velocity profile are preferably processed before the adjustment, wherein the measured velocity profiles are subjected to time averaging in particular. This yields more reliable velocity profiles for the subsequent model adjustment.

A respective statistical fluctuating variable, in particular the standard deviation, is preferably determined from the ascertained wall thrust stresses and/or the ascertained velocity profile, and compared with a prescribed limiting value for the fluctuating variable. This comparison is preferably used as the basis for selecting reliable measured data.

A suitable model can be selected by checking the boundary conditions used in the model. For example, it can be assumed that the velocity of the fluid at the wall is zero, i.e., that there is wall adhesion. Depending on whether curve adjustment is successful or not, this assumption can be accepted or rejected. The assumption that the velocity of the fluid at the wall differs from zero can e handled in like manner.

The boundary conditions can also be advantageously checked by counting failed iteration steps during the attempted adjustment of a model, wherein another model with different parameters and/or different boundary conditions is selected in particular when a preset number of iteration steps has been exceeded.

The used models are preferably selected from the following group of models:

-   -   Potency law model     -   Herschel-Bulkley model     -   Cross model

Other rheological models can also be used.

The used boundary conditions are preferably selected from the following group of boundary conditions:

-   -   Fluid velocity at the wall is zero, or presence of wall adhesion     -   Fluid velocity at the wall is not zero, or presence of wall         sliding     -   Yield point dipped below in an area of the fluid flow, or         stopper present in flow     -   Yield point not dipped below in any area of the fluid flow, or         stopper not present in flow     -   Flow state: laminar     -   Flow state: turbulent

The used models and boundary conditions are described in “Ultrasound Doppler Based In-Line Rheometry of Highly Concentrated Suspensions” by B. Ouriev, Diss. ETH No. 13523, Zürich 2000, or in “Rheological study of concentrated suspensions in pressure-driven shear flow using a novel in-line ultrasound Doppler method’ by B. Ouriev and E. J. Windhab, Experiments in Fluids 32 (2002).

At least some of the determined rheological parameters of the fluid can also be compared with values for this parameter that were ascertained in other ways. This makes it possible to additionally verify the results for the rheological parameters. The other method for determining the rheological parameters preferably involves measuring the viscosity in a rotational rheometer and/or in a capillary rheometer.

The static fluctuating variable, in particular the standard deviation, is best determined for the acquired velocity signals for each velocity channel (=location in velocity profile) and/or for the acquired pressure signals of each pressure measuring point. This information can be used among other things to tell whether the flowing fluid is in a turbulent or laminar flow state.

The arrangement according to the invention preferably has only one thrust stress sensor, which is situated in the at least one wall area, and is used to acquire a wall thrust stress in at least one area of the fluid in contact with the wall.

In one embodiment of the arrangement according to the invention that is especially preferred, since it is particularly compact, the thrust stress sensor, ultrasound transmitter and ultrasound receiver or ultrasound transceiver are arranged in the at least one wall area.

In another preferred embodiment, the arrangement according to the invention can have at least a first wall area and a second wall area, between which a fluid flowing in the device can respectively stream, and which contact and limit the fluid boundary at least in partial areas, wherein an ultrasound transmitter is preferably situated in the first wall area, and an ultrasound receiver in the second wall area. This makes it possible to use ultrasound waves that are not directionally reversed by 180°, but undergo only a relatively small directional change, when reflected/scattered by the particles entrained in the liquid. This ensures approximately the same running time or approximately the same path length in the flowing medium for all ultrasound waves received at the opposing ultrasound sensor. As a result, the traversed, flowing medium exercises approximately the same dampening effect on practically all ultrasound waves reflected/scattered between the ultrasound sensor and ultrasound receiver via particles in various interspersed fluid areas. However, one must remember that, while the absorption of received sound waves evens out as the deviation angle of the sound waves reflected/scattered on the moved particles decreases, this is necessarily accompanied on the one hand by a correspondingly reduced resolution during the localization of the respective reflecting fluid areas in step a) of the method according to the invention, in particular when using pulsed signals, and on the other hand by lower frequency shifts. However, the frequency shifts also increase as the flow velocities rise in the fluid, thereby at least offsetting the impact of the low deviation angle.

A first ultrasound transceiver is preferably situated in the first wall area, and a second ultrasound transceiver in the second wall area. This makes it possible to irradiate the flow with ultrasound waves “from left to right” and simultaneously “from right to left”, so that “left” and “right” measuring results can be obtained. This is particularly advantageous when the question is whether the certain asymmetries in the experimentally determined velocity distribution transverse to the direction of flow are only metrological artifacts or actual asymmetries in the real velocity distribution in the fluid. Such artifacts mimic an asymmetry in velocity distribution, and can generally be corrected by averaging the two distributions affected by the artifacts. If the averaged result is then still asymmetrical, this points to an actual asymmetry in the flow.

A first ultrasound transceiver and a first thrust stress sensor are preferably situated in the first wall area here as well, while a second ultrasound transceiver and second thrust stress sensor are arranged in the second wall area.

The wall area of the arrangement can be the interior wall of a tube or channel section that can be integrated into a line or channel for fluid transport, wherein the ultrasound transceiver and thrust stress sensor are preferably integrated in a compact ultrasound transceiver/thrust stress sensor measuring transducer unit. The arrangement can additionally incorporate a pressure sensor. This makes it possible to design not only each individual element of the arrangement like a “probe”, but the entire arrangement according to the invention, which ensures an even better “process accessibility” of the method according to the invention.

Additional advantages, features and possible applications of the invention can be gleaned from the following description of preferred exemplary embodiments, which are not to be regarded as limiting. Shown on:

FIG. 1 is a measuring arrangement according to prior art;

FIG. 2 is a measuring arrangement according to the invention; and

FIG. 3 to 7 in diagrammatic fashion is the procedure according to the invention fro processing and evaluating the measured values acquired with the measuring arrangement on FIG. 1.

FIG. 1 shows a tube section 1 in which a fluid 2 flows. The measuring arrangement on FIG. 1 comprises an ultrasound transceiver 3 as well as a pressure sensor 4 downstream and pressure sensor 5 upstream from the ultrasound transceiver 3.

According to the ultrasound Doppler method (UVP method), an ultrasound transceiver 3 sends out a narrow ultrasound wave US with a frequency f1 (practically flat wave or parallel beam) into the flowing fluid 2 transverse to the direction of fluid flow. The ultrasound wave US is reflected or scattered by moving particles that are entrained in the flowing fluid 2. The portion of the ultrasound wave US reflected or scattered back into the ultrasound transceiver 3 has a shifted frequency f2 owing to the particle motion (Doppler shift). This frequency shift provides information about the velocity of the particles or fluid in a specific fluid volume. The acquired varying frequency shifts are allocated to locations in the fluid where the frequency-shifting reflection or scattering takes place by measuring the running time between when the ultrasound wave was transmitted and received at the ultrasound transceiver 3. This is why pulsed ultrasound waves are used. The smaller the distances between the sequentially received reflected ultrasound pulses are in terms of time, and hence location, the greater the local resolution and number of acquired frequency shifts. The velocity profile can be determined in this manner.

The two pressure sensors 4 and 5 are used to measure a first static pressure P1 downstream and a second static pressure P2 upstream from the area of the fluid flow traversed by the ultrasound waves. The wall thrust stress in the fluid is then determined from this.

The viscosity function (shear viscosity) of the fluid can be determined by combining the fluid velocity distribution (“reaction of the fluid”) transverse to the direction of flow and the fluid wall thrust stress (“external influence on the fluid”).

In addition to determining the fluid-wall thrust stress and the fluid velocity profile, a suitable model for the viscosity function (shear viscosity) along with suitable boundary conditions for the flowing fluid are selected according to the invention.

FIG. 2 also shows a tube section 1 in which fluid 2 flows. The measuring arrangement on FIG. 2 comprises an ultrasound transceiver 3 as well as a thrust stress sensor 6 opposite the ultrasound transceiver 3 in the area in which the velocity profile is to be acquired.

According to the ultrasound Doppler method (UVP method), the ultrasound transceiver 3 is used to determine the velocity profile as already described on FIG. 1 here as well.

However, a thrust stress sensor 6 that enables a local determination of wall thrust stress in the fluid is here used instead of the two pressure sensors 4 and 5. The wall thrust stress is here determined directly and in the area where the velocity profile is also determined via UVP. While a “global”, indirect acquisition takes place in prior art (FIG. 2), in which the entire distance between the two pressure sensors is necessarily averaged, the arrangement according to the invention is used in the inventive method to rather perform a “local”, direct determination of the wall thrust stress. Therefore, an allocation between the actual values for the wall thrust stress (boundary condition) at the location of the acquired velocity profile and the velocity profile belonging to this boundary condition ends up taking place.

The fluid velocity distribution (“reaction of the fluid”) transverse to the direction of flow and the fluid wall thrust stress (“external influence on the fluid”) can here again be combined to determine the viscosity function (shear viscosity).

FIG. 3 diagrammatically shows the process for evaluating the measured thrust stress information to determine the wall thrust stress in the fluid. The geometry of the flow channel is input at 1). The thrust stress S is input at 2), while additional thrust stresses are input as needed at 3) and 4). Up to N different thrust stresses S1 to SN can (optionally!) be input. The input thrust stress value is filtered in a filter at 8) via a triggering that takes place at 9) in order to smooth out the signal. A wall thrust stress is then output at 5). The wall thrust stress distribution is determined at 6), and pressure fluctuations as a rule measured only at one location are determined at 7) as needed.

FIG. 4 diagrammatically shows the process for handling the unprocessed, “raw” velocity profiles before curve adjustment. Measured, unprocessed velocity profiles are input at 1). The fluid sound velocity measured for the examined fluid and prescribed sound frequency is input at 2). The values for the input velocity profiles are subjected to time averaging at 18) via a triggering that takes place at 19) to obtain averaged velocity profiles at 3). In addition, the parameters used fro the ultrasound Doppler method are input at 18), specifically the Doppler angle at 11), acoustic information at 12), the initial depth at 13), the channel distance at 14), the measuring window at 15), the pulse repetition rate at 16) and the used beam geometry at 17). The standard deviation for each velocity channel of the velocity profile is determined at 4), and then compared with a predetermined limiting value at 5). The actual initial depth, the actual penetration depth and the actual channel distance are then determined from this at 6), 7) and 8), respectively. Proceeding from these three values, reliable velocity data are then selected at 9) for subsequent calculations, which are finally prepared for the curve adjustment at 10).

FIG. 5 diagrammatically shows the process for selecting a suitable model, choosing reliable data and checking the boundary conditions for the flowing fluid. The velocity data for curve adjustment are prepared at 1). The determined standard deviation SMD is compared with a predetermined limiting value SMDL for standard deviation at 2).

If SMD is less than SMDL, a decision is made at 3) to use the SMD to adjust the data (curve adjustment). In this case, a curve adjustment is performed by method of least error squares at 4). This is used at 5) to monitor the axial symmetry of the flow profile, and at 6) to determine the maximum flow velocity. If SMD is greater than SMDL, the process of solving the boundary value problem is initiated at 18), and a warning signal is output at 20). The warning signal indicates that the boundary conditions have not been satisfied.

It is determined whether the wall velocity is zero at 7). If it is then confirmed that the velocity at the wall differs from zero at 11), the potency law model is loaded at 16) with the assumption of a wall sliding effect. By contrast, if it is denied at 11) that the velocity at the wall differs from zero, this statement (velocity at the wall is zero) is taken as a condition at 12). It is determined whether the maximum flow velocity is constant at 8), whether the pressure fluctuations are low at 9), i.e., whether the SMD is low, and whether the temperature difference along the flow channel (tube) is low or zero at 10).

If all conditions 8), 9) and 10) are confirmed or satisfied at 12), and if the velocity at the wall is zero, the potency law model is loaded at 13) as part of the approach to resolving the problem. If it is then determined at 14) that the flow index is greater than a set limit, the potency law model is loaded at 16) with the assumption of a wall sliding effect, and the model assuming a yield point is loaded at 17). By contrast, if it is determined at 14) that the flow index is less than a set value, the potency law model is loaded at 15). By contrast, if all conditions 8), 9) and 10) are denied or not satisfied at 12), and if the velocity at the wall differs from zero, the solution involving the potency law model is introduced at 18) to determine the flow index, while a repetition of the measurement is initiated at 19), and a warning signal is output at 20).

FIG. 6 diagrammatically shows the process for solving a boundary value problem via curve adjustment, in particular when considering the assumption of wall sliding during curve adjustment.

At 1), it is assumed that the maximum flow velocity is constant. At 2) and in the first iteration of 9), it is assumed that the velocity at the wall is zero. At 3), information from 1) and 2) is used to implement an adjustment using the potency law model. The flow index n of the potency law model of 7) is then set to n=1 at 4). Under this precondition along with the precondition of 1) and that stemming from 9) to the effect that the sliding velocity on the wall is zero, the theoretical velocity profile is calculated for n=1 at 5).

The input data are then adjusted at 6) by method of least error squares. The data for this purpose are input at 18), 19), 20), 21) and 22), specifically the actual penetration depth at 18), the actual penetration depth at 19), the actual channel distance at 20), the standard deviation exceeding the standard deviation limit at 21), and information about the axial symmetry of the flow profile from the axial symmetry controller at 22). A range of data is selected at 17) from the data input at 18) to 22). The flow index or criteria for adjustment quality are gleaned at 12) and 13) from the adjustment that took place at 6).

It is decided at 11) whether the flow index read at 12) is less than a lowest limiting value or not. If this is not the case, the flow index is incremented at 10), and again used with the potency law model at 3). An iterative process is then followed, wherein the steps 10), 3), 4), 5), 6), 12) and 13) are run through repeatedly. If the other case is still present at 11), specifically if the flow index is less than a lowest limiting value, the boundary value problem is resolved using other models at 8).

It is assumed at 7) that the fluid glides along the wall, meaning that the fluid velocity at the wall differs from zero. This assumption is used in conjunction with the potency law model at 4) to in turn calculate a corresponding theoretical profile at 5). The process for iteration then continues just as in the preceding paragraph.

It is decided at 16) whether the criteria for adjustment quality read at 13) exceed preset limiting values or not. If this is the case, an inquiry is made at 14) as to whether the number of iterations exceeds a preset number or not. If this is the case, the sliding velocity at the wall is set to zero at 9) in a first iteration step, and iteration is repeated at 5). Otherwise, the iteration is continued. The process for iteration then continues just as in the preceding paragraphs. If it is decided at 16) that the adjustment criteria do not exceed the preset limiting values, the arguments (e.g., flow index, sliding velocity, radius of stopper, etc.) are output at 15), and one continues on to 23).

An inquiry is made at 23) whether the sliding velocity at the wall is zero or not. If it is zero, the volumetric flow rate, wall shear velocity and shear velocity distribution are calculated at 24), 25) and 26) form the adjustment to the velocity profile. If the sliding velocity at the wall is not zero, the respective volumetric flow velocity, wall shear velocity and shear velocity distribution are calculated in an analogous manner at 27), 28) and 29) from the adjustment to the velocity profile assuming wall sliding. Based on the variables calculated at 24) to 26) or at 27) to 29), the wall shear viscosity is then calculated at 32), and the shear viscosity function at 33) (e.g., its distribution along a direction transverse to the flow).

FIG. 7 diagrammatically shows the process for determining the flow state. The potency law model, Herschel-Bulkley model, Cross model or other models are used as the basis at 1), 2), 3) and 4), respectively. The boundary value problem is again solved from this at 7), and the rheological variables are output at 9).

It is decided at 15) whether the shear viscosity calculated from the shear viscosity distribution is less than a preset viscosity limit input by the user, which was used via off-line and/or on-line reference measurements, e.g., utilizing a rotational rheometer, a capillary rheometer or some other rheometer. If the shear viscosity is less than the limiting value, it is decided that a turbulent flow state per 14) is present. If the shear viscosity is greater than or equal to the limiting value, it is decided that a laminar flow according to 13) is present.

An approach to solving the turbulent flow state is used at 6). This approach differs from that used for the laminar flow state only by the adjustment model, which has a similar form, but uses other values for the parameters, e.g., for the flow index. The “turbulent” flow index is then calculated at 6) and used at 11).

It is decided at 12) whether the flow index is less than a lowest limiting value or not. If the flow index is lower, it is decided that a turbulent flow state according to 14) is present. If the flow index is equal to or greater than the lowest limiting value, and based upon an evaluation of the variables for SMD, viscosity, flow index and maximum velocity, it is decided that a laminar flow state according to 13) or a turbulent flow state according to 14) is present, and the approach for the turbulent flow is used at 6).

It is assumed at 10) that wall sliding is present. The approach taking into account wall sliding is used at 5). This yields a value for wall sliding at 8) along with other rheological parameters at 9).

The standard deviation SMD is used as the basis for each velocity channel at 18). It is decided at 16) whether the SMD exceeds a maximum limit or not. If the SMD does exceed this maximum limit, it is in turn decided at 12) whether the flow index is less than the lowest limiting value or not. If yes, the turbulent state according to 14) is present. If no, the laminar flow state according to 13) is present.

The standard deviation SMD of the current n-th adjustment to the unprocessed “raw’ pressure signal is used as the basis at 19). It is decided at 17) whether pressure fluctuations are present or not. If pressure fluctuations are present and if permitted by an evaluation of viscosity, flow index and maximum velocity, it is decided that a turbulent flow according to 14) is present, and the approach for the turbulent flow is used at 6). If no pressure fluctuations are present, it is decided for the laminar state, and the approach for the laminar state is used at 7).

In sum, it can be stated that flow types can essentially be divided into the following:

-   -   laminar flow with stopper (highly viscous material, e.g., highly         concentrated suspension)     -   laminar flow without stopper (both with flow index n>1, i.e.,         dilatant or shear-thickening material, and with flow index n<1,         i.e., structurally viscous or shear-diluting material)     -   turbulent flow (low-viscous material, e.g., weakly concentrated         suspension).

The (“smoothened”) global flow profile of the turbulent flow transverse to the tube axis at which the local velocity fluctuations can be filtered out can be described analogously to the high-viscous stopper flow by the Herschel-Bulkley model, for example. 

1. A method for determining the rheological parameters of a flowing fluid, in particular a suspension or emulsion, wherein the fluid flow is limited at least in partial areas by a wall in contact with the fluid, and wherein the method comprises the following steps: a) sending into the fluid flow an ultrasound signal transmitted from an ultrasound transmitter at least at one prescribed first frequency f1 at an angle θ differing by 90° relative to the direction of flow; b) receiving into an ultrasound receiver an ultrasound signal reflected by particles entrained in the fluid in respective fluid areas with at least one second frequency f2 that is characteristic for the respective fluid area and shifted relative to the frequency f1 by a respective frequency shift Δf; c) acquiring the local wall thrust stress in at least an area of the fluid contacting the wall; d) calculating the at least one frequency shift Δf using the at least one first frequency f1 and the at least one second frequency f2; e) allocating the respective frequency shift Δf to a respective fluid area using the respective running time of the ultrasound signal between the point of transmission from the ultrasound transmitter and point of reception by the ultrasound receiver; f) calculating the fluid velocity of the respective fluid area in which the reflecting/particles are entrained using the respective frequency shift; and g) calculating rheological parameters of the fluid using the wall thrust stress of the fluid acquired in the at least one local wall area and the calculated fluid velocity of the respective local fluid areas of the flowing fluid.
 2. The method according to claim 1, wherein the transmitted ultrasound signal is a signal with several discrete first frequencies (f1, f1′, f1″, . . . ), and the received ultrasound signal is at least a second signal, each with several discrete second frequencies (f2, f2′, f2″, . . . ), which are shifted relative to the respective first frequencies (f1, f1′, f1″, . . . ) by a respective frequency shift Δf characteristic for the respective fluid area.
 3. The method according to claim 1, wherein the transmitted ultrasound signal is a signal with a first frequency spectrum (FS1), and the received ultrasound signal is at least one second signal with a respective second frequency spectrum (FS2), which is shifted relative to the first frequency spectrum (FS1) by a respective frequency shift Δf characteristic for the respective fluid area.
 4. The method according to claim 1, wherein the transmitted and received ultrasound signals are each pulsed signals.
 5. The method according to claim 4, wherein the pulsed signals each have a constant carrier frequency.
 6. The method according to claim 1, wherein the transmitted and received ultrasound signals are each continuous signals.
 7. The method according to claim 1, wherein the wall thrust stress is determined in step c) in only one area of the fluid in contact with the wall.
 8. The method according to claim 1, wherein the ultrasound signal radiated into the fluid is transmitted and the reflected ultrasound signal is received at the same location.
 9. The method according to claim 1, wherein a local velocity profile transverse to the direction of flow is generated from the fluid velocities of the fluid areas of the fluid calculated in step f).
 10. The method according to claim 9, wherein the viscosity function (shear viscosity) of the fluid is determined from the local velocity profile calculated in step f) and the local wall thrust stress acquired in step c) in at least one area of the fluid in contact with the wall.
 11. The method according to claim 7, wherein a suitable model is adjusted by iteratively adapting a modellable theoretical velocity profile to a measured velocity profile.
 12. The method according to claim 11, wherein the measured velocity profiles are processed before the adjustment.
 13. The method according to claim 12, wherein processing involves a time averaging of the measured velocity profiles.
 14. The method according to claim 1, wherein a respective statistical fluctuating variable, in particular the standard deviation, is determined from the measured wall thrust stresses and/or the ascertained velocity profiles, and compared with a preset limiting value for the fluctuating variable.
 15. The method according to claim 14, wherein the comparison is used as the basis for selecting reliable measured data.
 16. The method according to claim 1, wherein a suitable model is selected by checking the boundary conditions used in the model.
 17. The method according to claim 16, wherein the boundary conditions are checked by counting failed iteration steps during the attempted adjustment of a model.
 18. The method according to claim 17, wherein another model with other parameters and/or other boundary conditions is selected if a preset number of iteration steps has been exceeded.
 19. The method according to claim 1, wherein the used models are selected from the following group of models: Potency law model Herschel-Bulkley model Cross model
 20. The method according to claim 1, wherein the used boundary conditions are selected from the following group of boundary conditions: fluid velocity at the wall is zero, or presence of wall adhesion fluid velocity at the wall is not zero, or presence of wall sliding yield point dipped below in an area of the fluid flow, or stopper present in flow yield point not dipped below in any area of the fluid flow, or stopper not present in flow flow state: laminar flow state: turbulent
 21. The method according to claim 1, wherein at least some of the determined rheological parameters of the fluid are also compared with values for this parameter that were ascertained in other ways.
 22. The method according to claim 21, wherein the other way of determining the rheological parameters is to measure the viscosity in a rotational rheometer and/or in a capillary rheometer.
 23. The method according to claim 14, wherein the statistical fluctuating variable, in particular the standard deviation, is determined at least for: the acquired velocity signals for each velocity channel (=location in velocity profile); the locally measured thrust stress signals; local pressure signals at pressure measuring points.
 24. A device for determining the rheological parameters of a flowing fluid using the method according to claim 1, wherein the device comprises: at least one wall area that contacts and borders a fluid flowing in the device at least in partial areas of the fluid interface; an ultrasound transmitter for sending an ultrasound signal with at least one preset frequency f1 at an angle θ differing by 90° relative to the direction of flow of a fluid flowing in the device; an ultrasound receiver for receiving an ultrasound signal with at least one frequency f2 that can be shifted relative to frequency f1 by a frequency shift Δf; at least one thrust stress sensor for detecting a wall thrust stress in at least one area of the fluid in contact with the wall; one computer and processor for calculating frequency differences and allocating a specific frequency difference to a respective fluid area using a respective time difference between the transmission and reception of an ultrasound signal; for calculating the fluid velocity of a respective fluid area using the respective frequency shift; and for calculating rheological parameters of the fluid using the acquired wall thrust stress of the fluid and the calculated fluid velocity of the respective fluid areas of the flowing fluid.
 25. The device according to claim 24, wherein it only has one thrust stress sensor arranged in at least one wall area for determining a wall thrust stress in at least one area of the fluid in contact with the wall.
 26. The device according to claim 24, wherein the ultrasound transmitter and ultrasound receiver are combined into a single ultrasound transceiver.
 27. The device according to claim 24, wherein the thrust stress sensor, ultrasound transmitter and ultrasound receiver or ultrasound transceiver are arranged in the at least one wall area.
 28. The device according to claim 24, wherein it has at least one first wall area and one second wall area, between which a fluid flowing in the device can respectively stream, and which contact and limit the fluid boundary at least in partial areas.
 29. The device according to claim 28, wherein an ultrasound transmitter is preferably situated in the first wall area, and an ultrasound receiver in the second wall area.
 30. The device according to claim 28, wherein a first ultrasound transceiver is situated in the first wall area, and a second ultrasound transceiver in the second wall area.
 31. The device according to claim 18, wherein a first ultrasound transceiver and a first thrust stress sensor are situated in the first wall area, while a second ultrasound transceiver and second thrust stress sensor are arranged in the second wall area.
 32. The device according to claim 24, wherein the wall area is the interior wall of a tube or channel section that can be integrated into a line or channel for fluid transport.
 33. The device according to claim 26, wherein the ultrasound transceiver and thrust stress sensor are integrated in a compact ultrasound transceiver/thrust stress sensor measuring transducer unit.
 34. The device according to claim 26, further comprising a pressure sensor. 